1. Field of the Invention
The present invention relates to wireless communication systems, and more particularly, to a method and apparatus for communicating messages over a digital control channel in a cellular radio system.
2. History of the Prior Art
In a typical cellular radio system, a geographical area, e.g., a metropolitan area, is divided into several cells, each of which is served by a base station having a limited radio coverage area. The base stations are connected to a mobile services switching center (MSC) which is, in turn, connected to the landline public switched telephone network (PSTN). Each user (mobile subscriber) in the cellular radio system is provided with a portable, pocket, hand-held or car-mounted device (mobile station) which communicates voice and/or data with a nearby base station and the MSC. The MSC facilitates communications, e.g., switches calls and controls signalling, between the mobile station and other mobile stations in the system or landline telephones in the PSTN. FIG. 1 illustrates the architecture of a conventional cellular radio system built according to the Advanced Mobile Phone Service (AMPS) standard.
In FIG. 1, an arbitrary geographic area may be seen divided into a plurality of contiguous radio coverage areas, or cells, C1-C10. While the system of FIG. 1 is, for illustration purposes, shown to include only ten cells, the number of cells may be much larger in practice. Associated with and located in each of the cells C1-C10 is a base station designated as a corresponding one of a plurality of base stations B1-B10. Each of the base stations B1-B10 includes a plurality of channel units, each comprising a transmitter, a receiver and a controller, as is well known in the art. In FIG. 1, the base stations B1-B10 are located at the center of the cells C1-C10, respectively, and are equipped with omni-directional antennas transmitting equally in all directions. In this case, all the channel units in each of the base stations B1-B10 are connected to one antenna. However, in other configurations of the cellular radio system, the base stations B1-B10 may be located near the periphery, or otherwise away from the centers of the cells C1-C10 and may illuminate the cells C1-C10 with radio signals directionally. For example, the base station may be equipped with three directional antennas, each one covering a 120 degrees sector cell as shown in FIG. 2. In this case, some channel units will be connected to one antenna covering one sector cell, other channel units will be connected to another antenna covering another sector cell, and the remaining channel units will be connected to the remaining antenna covering the remaining sector cell. In FIG. 2, therefore, the base station serves three sector cells. However, it is not always necessary for three sector cells to exist and only one sector cell needs to be used to cover, for example, a road or a highway.
Returning to FIG. 1, each of the base stations B1-B10 is connected by voice and data links to a mobile switching center (MSC) 20 which is, in turn, connected to a central office (not shown) in the public switching telephone network (PSTN) or a similar facility, e.g., an integrated system digital network (ISDN). The relevant connections and transmission modes between the mobile switching center MSC 20 and the base stations B1-B10, or between the mobile switching center MSC 20 and the PSTN or ISDN, are well known to those of ordinary skill in the art and may include twisted wire pairs, coaxial cables, fiber optic cables or microwave radio channels operating in either analog or digital mode. Further, the voice and data links may either be provided by the operator or leased from a telephone company (telco).
With continuing reference to FIG. 1, a plurality of mobile stations M1-M10 may be found within the cells C1-C10. Again, while only ten mobile stations are shown in FIG. 1, the actual number of mobile stations may be much larger in practice and will invariably exceed the number of base stations. Moreover, while none of the mobile stations M1-M10 may be found in some of the cells C1-C10, the presence or absence of the mobile stations M1-M10 in any particular one of the cells C1-C10 depends on the individual desires of each of the mobile subscribers who may travel from one location in a cell to another or from one cell to an adjacent or neighboring cell. Each of the mobile stations M1-M10 includes a transmitter, a receiver, a controller and a user interface, e.g., a telephone handset, as is well known in the art. Each of the mobile stations M1-M10 is assigned a mobile identification number (MIN) which, in the United States, is a digital representation of the telephone directory number of the mobile subscriber. The MIN defines the subscription of the mobile subscriber on the radio path and is sent from the mobile station to the MSC 20 at call origination and from the MSC 20 to the mobile station at call termination. Each of the mobile stations M1-M10 is also identified by an electronic serial number (ESN) which is a factory-set, "unchangeable" number designed to protect against the unauthorized use of the mobile station. At call origination, for example, the mobile station will send the ESN to the MSC 20. The MSC 20 will compare the received ESN to a "blacklist" of the ESNs of mobile stations which have been reported to be stolen. If a match is found, the stolen mobile station will be denied access.
Each of the cells C1-C10 is allocated a subset of the radio frequency (RF) channels assigned to the entire cellular system by the concerned government authority, e.g., the Federal Communications Commission (FCC) in the United States. Each subset of RF channels is divided into several voice or speech channels which are used to carry voice conversations, and at least one paging/access or control channel which is used to carry supervisory data messages, between each of the base stations B1-B10 and the mobile stations M1-M10 in its coverage area. Each RF channel comprises a duplex channel (bidirectional radio transmission path) between the base station and the mobile station. The RF consists of a pair of separate frequencies, one for transmission by the base station (reception by the mobile station) and one for transmission by the mobile station (reception by the base station). Each channel unit in the base stations B1-B10 normally operates on a preselected one of the radio channels allocated to the corresponding cell, i.e., the transmitter (TX) and receiver (RX) of the channel unit are tuned to a pair of transmit and receive frequencies, respectively, which is not changed. The transceiver (TX/RX) of the each mobile station M1-M10, however, may tune to any of the radio channels specified in the system.
Depending on capacity needs, one cell may have 15 voice channels, while another may have over a 100 voice channels, and corresponding channel units. Generally speaking, however, there is only one control channel (CC) in each omnidirectional or sector cell served by a base station, i.e., a base station serving an omnidirectional cell (FIG. 1 ) will have one control channel unit while a base station serving three sectors cells (FIG. 2) will have three control channel units. The RF (control and voice) channels allocated to any given cell may be reallocated to a distant cell in accordance with a frequency reuse pattern as is well known in the art. To avoid radio interference, all radio channels in the same cell will operate on different frequencies and, furthermore, the radio channels in any one cell will operate on a set of frequencies which is different from that used in any neighboring cell.
When in the idle state (not in the conversation state), each of the mobile stations M1-M10 tunes to and then continuously monitors the strongest control channel (generally, the control channel of the cell in which the mobile station is located at that moment) and may receive or initiate a telephone call through the corresponding one of the base stations B1-B10 which is connected to the mobile switching center MSC 20. When moving between cells while in the idle state, the mobile station will eventually "lose" radio connection on the control channel of the "old" cell and tune to the control channel of the "new" cell. The initial tuning to, and the change of, control channel are both accomplished automatically by scanning all the control channels in operation in the cellular system (in the United States, there are 21 "dedicated" control channels in each AMPS system, i.e., their TX/RX frequencies are predefined and cannot be changed, which means that the mobile station has to scan a maximum number of 21 channels to find the "best" control channel). When a control channel with good reception quality is found, the mobile station remains tuned to this channel until the quality deteriorates again. In this manner, all mobile stations are always "in touch" with the system.
While in the idle (standby) state, each of the mobile stations M1-M10 continuously determines whether a page message addressed to it has been received over the control channel. When, for example, an ordinary (landline) subscriber calls one of the mobile subscribers, the call is directed from the PSTN to the MSC 20 where the dialed number is analyzed. If the dialed number is validated, the MSC 20 requests some or all of the base stations B1-B10 to page the called mobile station throughout their corresponding cells C1-C10. Each of the base stations B1-B10 which receive the request from the MSC 20 will then transmit over the control channel of the corresponding cell a page message containing the MIN of the called mobile station. Each of the idle mobile stations M1-M10 will compare the MIN in the page message received over the control channel being monitored with the MIN stored in the mobile station. The called mobile station with the matching MIN will transmit a page response over the control channel to the base station which forwards the page response to the MSC 20.
Upon receiving the page response, the MSC 20 selects an available voice channel in the cell from which the page response was received and requests the base station in that cell to order the mobile station via the control channel to tune to the selected voice channel (the MSC keeps a list of all of the channels in its service area and their status, i.e., free, busy, blocked, etc., at any time). A through-connection is established once the mobile station has tuned to the selected voice channel. When, on the other hand, a mobile subscriber initiates a call, e.g., by dialing the telephone number of an ordinary subscriber and pressing the "send" button on the telephone handset in the mobile station, the MIN and ESN of the mobile station and the dialed number are sent over the control channel to the base station and forwarded to the MSC 20 which validates the mobile station, assigns a voice channel and establishes a through-connection for the conversation as before.
If the mobile station moves between cells while in the conversation state, a "handoff" of the call from the old base station to the new base station will take place. The MSC selects an available voice channel in the new cell and then orders the old base station to send to the mobile station on the current voice channel in the old cell a handoff message which informs the mobile station to tune to the selected voice channel in the new cell. The handoff message is sent in a "blank and burst" mode which causes a short but hardly noticeable break in the conversation. Upon receipt of the handoff message, the mobile station tunes to the new voice channel and a through-connection is established by the MSC via the new cell. The old voice channel in the old cell is marked idle in the MSC and may be used for another conversation.
In addition to call originations and page responses, an AMPS mobile station may access the cellular system for registrations. Two types of registrations are possible in AMPS: (i) periodic registration which is based on time or, more specifically, on the REGID value ("current time") and REGINCR value ("registration period") transmitted by the base station and the NXTREG value ("wake-up time") stored in the mobile station, and (ii) system area registration which is based on location or, more specifically, on the system identification (SID) transmitted in the serving cellular system. Periodic registration may be used to determine whether a mobile station is active (within radio range and switched on) or not in a cellular system. System area registration may be used to determine when a mobile station has crossed the border from one cellular system to another.
Upon receipt of a REGID message on the forward control channel (base station to mobile station), if registration is enabled in the serving cellular system, the mobile station compares the REGID value to the NXTREG value and compares the last received SID value with the value of the SID of the cellular system in which the mobile station last registered. If either the value of REGID is greater or equal to the value of NXTREG indicating that periodic registration is due, or the value of the last received SID is different than the value of the last stored SID indicating that the mobile station has travelled from one cellular system to another since the last successful registration, the mobile station will automatically send a registration access message over the reverse control channel (mobile station to base station) and will update the NXTREG value with the sum of the last received REGID value and REGINCR value, after receipt of a registration acknowledgement message on the forward control channel (the mobile station also updates the NXTREG value after each call origination or page response).
The conventional AMPS system described above uses frequency division multiplexing (FDM) to carry telephone conversations and control information over the voice and control channels. As mentioned, the available frequency spectrum is divided among the cells in the system. In each cell, the voice (analog) signals and data (digital) signals form the input signals to a transmitter (in the base station or the mobile station) which generates a sinusoidal carrier wave having a constant frequency corresponding to one of the frequencies allocated to the cell. The transmitter uses the input signals to modulate a characteristic (amplitude, frequency or phase) of the carrier wave prior to radio transmission. The modulated carrier occupies a relatively narrow region (channel bandwidth) of the spectrum about a nominal center frequency (the unmodulated carrier frequency). Generally, frequency modulation is used so that the carrier frequency at any instant of time is varied (increased or decreased) in proportion to the amplitude of the input signal at that instant. The resulting deviation of the modulated carrier wave frequency about the unmodulated (center) frequency is normally limited within a certain bandwidth, e.g., 30 KHz, to avoid overlapping adjacent RF channels and causing adjacent channel interference.
In the conventional AMPS system, therefore, an analog speech signal modulates the carrier wave used for transmission over the RF channel. The AMPS system uses analog frequency modulation (FM) and is a single-channel-per-carrier (SCPC) system, i.e., one voice circuit (telephone conversation) per RF channel. Recent developments, however, have ushered in a new digital era for cellular communications. The main driving force behind the switch to digital has been the desire to increase spectrum efficiency to meet the ever-increasing demands on system capacity. By encoding (digitizing and compressing) and multiplexing speech from several voice circuits prior to modulation and transmission, a single RF voice channel may be shared by several digital speech channels instead of being occupied by only one analog speech channel. In this manner, the channel capacity and, consequently, the overall system capacity, may be increased dramatically without increasing the bandwidth of the voice channel. As a corollary, the cellular radio system is able to serve a substantially greater number of mobile stations at a significantly lower cost, e.g. , a smaller number of channel units (transceivers) required in the base stations. Furthermore, the digital format facilitates integration of the cellular system with the emerging digital network.
In the United States, the effort to "go digital" has been spearheaded by the Electronics Industries Association (EIA) and the Telecommunication Industry Association (TIA) which have formulated an interim standard for the air interface in digital cellular systems. This EIA/TIA interim standard is known as the "Dual-Mode Mobile Station--Base Station Compatibility Standard" and is designated as "IS-54" (copies of the various revisions of IS-54 may be obtained from the Electronics Industries Association; 2001 Pennsylvania Avenue, N.W.; Washington, D.C. 20006). The term "dual-mode" refers to the capability of the system to operate in either an analog or digital mode. The analog mode of operation draws on the EIA/TIA 553 standard which is based on the AMPS standard. The digital mode of operation relies on time division multiplexing (TDM) techniques similar to those which have long been used in the land-line telephone network to carry multiple telephone conversations simultaneously over one physical channel (code division multiplexing (CDM) has also been proposed for cellular systems, but the current IS-54-B (Rev. B) specification uses TDM).
In the wire-line telephone network, analog speech signals transmitted by local telephone subscribers over separate analog channels to the local telephone company (telco) central office are sequentially sampled and the amplitudes of the samples are quantized and then encoded into binary numbers represented by constant amplitude pulses in a process called pulse code modulation (PCM). A predetermined number of PCM channels (digital speech channels) are transmitted in a series of frames, each containing a burst of information (coded samples) from each of the PCM channels. The bursts from the different PCM channels occupy different time slots (time intervals) in each frame transmitted on the physical channel, e.g. , copper wire plant. Most long-distance telephone calls are transmitted through the switching hierarchy using TDM. This technique can also be applied to transmissions on the RF channels of a cellular radio system.
An RF channel operating in TDM is divided into a series of repeating time slots each containing a burst of information from a different data source, e.g. a source coder for a voice channel. The time slots are grouped into frames of a predetermined duration. The number of time slots per frame varies depending on the number digital channels sought to be accommodated on the RF channel given the coding rates of the digital channels, the modulation level and the bandwidth of the RF channel. Each slot in a frame normally represents a different digital channel. The length of each TDM frame on the RF channel, therefore, is the minimum amount of time between two repeating time slots which are used by the same digital channel (assigned to the same user).. In other words, each TDM frame consists of no more than one s lot for each user.
According to IS-54, each digital TDM RF channel can carry from three to six digital speech channels (three to six telephone conversations) depending on the source rate of the speech coder used for each digital channel (the modulation level and channel bandwidth are set in IS-54). The speech coder for each digital traffic channel (DTC) can operate at either full-rate or half-rate (full-rate speech coders are expected to be used in the near future until half-rate coders are developed which produce acceptable speech quality). A full-rate DTC requires twice as many time slots in a given time period as a half-rate DTC. In IS-54, each TDM RF channel can carry up to three full-rate DTCs or six half-rate DTCs.
The TDM RF channel frame structure for IS-54 is shown in FIG. 3. Each "frame" on the TDM RF channel comprises six equally sized time slots (1-6) and the length of the frame is 40 ms (25 frames per second). Each full-rate DTC uses two equally spaced slots of the frame shown in FIG. 3, i.e. , slots 1 & 4, or slots 2 & 5, or slots 3 & 6. When operating at full-rate, the TDM RF channel may be assigned to three users (A-C), i.e., user A is assigned to slots 1 & 4; user B is assigned to slots 2 & 5; and user C is assigned to slots 3 & 6 of the "frame" shown in FIG. 3 (for full-rate, therefore, each TDM frame actually consists of three slots and not six slots, and is 20 ms long and not 40 ms long). Each half-rate DTC uses one time slot of the frame shown in FIG. 3. At half-rate, the TDM RF channel may be assigned to six users (A-F) with each of the users A-F being assigned to one of the six slots of the frame shown in FIG. 3 ( for half-rate, each TDM frame actually consists of six slots and coincides with the definition of "frame" in IS-54).
Hence, unlike an analog FDM cellular system in which the base station and the mobile station transmit and receive continuously over an RF channel, a TDM cellular system operates in a buffer and burst discontinuous transmission mode. Each mobile station transmits (and receives) in an assigned slot on the RF channel. At full rate, for example, the mobile station of user A would transmit on slot 1, hold for slot 2 receive in slot 3, transmit in slot 4, hold for slot 5, and transmit in slot 6, and then repeat the cycle (the transmit and receive slots are offset from each other to avoid using duplexer circuitry which would otherwise be needed to allow the transmitter and receiver at the mobile station to operate simultaneously). The mobile station, therefore, transmits (or receives ) in a fraction of the time (one third for full rate and one sixth for half-rate) and can be switched off to save power the rest of the time.
The present IS-54 standard, however, is not a fully digital standard but a hybrid analog-digital standard which is intended to be followed in the transition phase from analog to digital where the mobile stations in operation will constitute a mixture of new dual-mode mobile stations and old strictly analog mobile stations. More specifically, the IS-54 standard provides for both analog speech channels in the tradition of AMPS and digital speech channels which are configured in the frame format shown in FIG. 3. At call set-up, the dual -mode mobile stations may be assigned either an analog voice channel (an entire carrier frequency) or, alternatively, a digital traffic channel (a repeating time slot on a carrier frequency). The analog-only mobile stations, however, can only be assigned an analog voice channel.
The continued need to serve existing analog-only mobile stations has also led to the specification in IS-54 of an analog control channel which has been inherited from the prior AMPS, or equivalently, EIA/TIA 553 standard. According to IS-54, the forward (paging) analog control channel on the down-link from the base station to the mobile stations carries a continuous data stream of messages (words) in a particular format. The reverse (access) analog control channel on the up-link from the mobile stations to the base station, however, is a random access channel which is used on a contention basis for transmission of call origination, page response and registration messages. A busy-idle bit transmitted on the forward control channel (FOCC) indicates the current status (availability) of the reverse control channel (RECC), i.e. , the RECC is busy if the busy-idle bit is equal to "0" and idle if the busy-idle bit is equal to "1."
The format of the FOCC specified in IS-54 is shown in FIG. 4. Several different types ( functional classes) of messages may be transmitted on the FOCC: (i) system parameter overhead message (SPOM), (ii) global action overhead message (GAOM), (iii) registration identification message (REGID), (iv) mobile station control message, e.g., paging message, and (v) control-filler message. The SPOM, GOAM and REGID are overhead messages which are intended for use by all mobile stations in the coverage area of the base station. Overhead messages are sent in a group called an overhead message train (OMT). The first message of each OMT must always be the SPOM which is transmitted every 0.8.+-.0.3 seconds.
The SPOM consists of two words which contain information about the serving cellular system including the system identification (SID) and control bits REGH and REGR which indicate whether registration is enabled for home stations and roaming stations, respectively (a home station is a mobile station which is operating in the cellular system from which service is subscribed while a roaming station is a mobile station which is operating in a cellular system other than the one from which service is subscribed). The GOAM or REGID consists of one word which is appended at the end of the SPOM and sent on an as-needed basis. Any number of global action messages may be appended to a SPOM as desired. The types of global action messages include rescan paging channels and registration increment (REGINCR) messages (REGINCR and REGID messages control the frequency of periodic registrations of mobile stations with the serving cellular system). When sent, the REGID message must be appended to the SPOM or, if any global action messages are sent, to the last GOAM in the OMT.
While the SPOM, GOAM and REGID are broadcast for use by all mobile stations listening to the forward control channel (FOCC), the mobile station control message, e.g., paging message, is directed to a specific mobile station (specific MIN). Other examples of mobile station control messages include analog voice channel or digital traffic channel (full-rate or half-rate) assignment messages and orders to change transmit power level. The mobile station control message consists of from one to four words. The control-filler message consists of one word which is sent whenever there is no message to be sent on the FOCC, i.e., to fill gaps between different messages or between blocks of a multi-word message.
The format of the forward analog control channel specified in IS-54 and shown in FIG. 4 is largely inflexible and not conducive to the objectives of modern cellular telephony including the extension of mobile station battery life. Specifically, the time interval between SPOM transmissions is fixed and the order in which overhead and control messages are appended to the SPOM is also rigid. While the cellular system can control the frequency of transmission of most overhead messages (only the SPOM needs to be included in each OMT), an idle mobile station which has tuned to the FOCC must repeatedly read all the messages in each OMT (except, for example, when a GOAM instructs the mobile station to rescan paging channels) not only the paging messages, even though the information contained in the overhead messages in the current OMT may not have changed from the previous OMT. Too often, therefore, the mobile station updates its memory with the same information which is already stored there. Battery power is wasted during these read cycles without any commensurate benefit to the operation of the mobile station.
In light of these drawbacks and shortcomings of the prior art analog control channel (ACC), it is an object of the present invention to provide a digital control channel (DCC) which may carry message types similar to those carried on the ACC, but in which the frequency of message transmission by the base station is mostly decoupled from the frequency of message reading by the mobile station. In other words, some types of messages may be transmitted more frequently than others but the mobile station does not have to read every message transmitted on the DCC.
For example, a mobile station which has just locked on to the DCC may need to obtain, as quickly as possible, all relevant information about the current serving system, e.g., ownership (is it a private system?), service profile (can it handle a particular data service?), system parameters (what is the maximum mobile station transmit power?), etc. This overhead information, therefore, may be sent as often as possible without unduly limiting the capacity of the DCC to carry other messages, e.g., paging messages. However, most of this overhead information does not change very often and it would be a waste of battery power for this information to be read too often. Hence, once the mobile station has read the overhead information, the mobile station will not read it again until the mobile station receives an indication that it has changed. This results in significant savings of battery power in the mobile station.
It is another object of the present invention to provide a DCC which enables a mobile station in idle mode to read a minimum amount of information from the DCC during predetermined periods of time, and to enter into a "sleep" mode at all other times. In this regard, the mobile station is allowed as short of a period as possible to read paging messages before returning to sleep mode. During sleep mode, most electronic circuits in the mobile station are shut off and there is minimal drain of battery power. In this manner, the battery life may be extended from, for example, 13 hours to 100 hours before recharging of the battery becomes necessary. The proportion of time spent reading page messages to the time spent in sleep mode is controllable and represents a tradeoff between call set-up delay and battery power consumption.
It is yet another object of the present invention to provide a flexible DCC format which is adaptable to a hierarchial cell structure consisting of both "macro" (large radius) cells and "micro" (small radius) cells. In a hierarchial cell structure, a mobile station may change cells much more often than in present macro-cell oriented systems. It is important that frequent cell selection and reselection does not hamper the ability of the mobile station to receive pages or place calls. The present invention allows for fast cell selection and reselection by transmitting overhead messages on a frequent basis while still providing for efficient sleep mode operation. The high repetition frequency of overhead messages allows mobile stations which are about to lock onto a new cell to quickly find the paging channel and the other parameters required for system access.
It is a further object of the present invention to provide the ability to adjust DCC capacity in each cell to meet the usage requirements in that cell, i.e. , the expected number of pages and accesses per second.
It is another object of the present invention to provide a DCC which facilitates the integration of the mobile network with the ever-growing portfolio of ISDN services.
It is also an object of the present invention to provide a DCC which may be easily implemented within the existing framework of IS-54.